Heavy equipment provides the muscle that keeps America running, shouldering most of the load for everything from building critical infrastructure to disposing of society’s waste. Regardless of the job it does do, however, each machine has a common need for prompt, efficient, on-site service and repair, which is usually supplied by a technician driving a fully equipped service truck.

Traditionally, the service truck is equipped with an air compressor to power impact wrenches and other tools, a generator to support work lights and welding operations, and a hydraulic crane to handle tires, wheels, and other large mechanical components. Each, in turn, is powered by the truck’s diesel engine via the power takeoff (PTO).

“What that means in practical terms,” notes Rick Beeson, of Miller Electric Mfg. Co., Appleton, Wis., “is that once a service truck reaches the job site, its engine will be running to power the compressor and hydraulic pump for as long as it takes to complete the job, while a secondary unit is used to provide auxiliary and welding power. A diesel engine with enough power to move a multi-ton truck will be used to run components that easily could be powered by the engine of a mid-size garden tractor.

“In today’s world, the maintenance and operating costs involved, not to mention the fuel used and emissions being generated, are simply unacceptable. Because our equipment is used on many service trucks, we accepted the challenge of finding a better solution to the need, and, based on the response from the field, I think we have.” Beeson is referring to Miller’s new EnPak Mechanic series integrated power system for service trucks. This completely self-contained unit is designed to fit into a truck’s bed or on the side of the body. Its 27-hp diesel engine powers a generator, a screw-type air compressor, and a hydraulic pump.

The EnPak uses a digital power management system to match engine output to load requirements. The variable displacement feature of the hydraulic pump plays a key role in matching engine power to the total load placed on the system. By understanding how users operate a crane, Miller officials concluded that an opportunity existed to match the power requirement of the hydraulic system to a more compact engine that uses less fuel and creates a better working environment. Beeson added: “Experienced operators just don’t move heavy loads quickly regardless of how much power is available, and we used that knowledge in designing the system.”

Choosing the optimal concept “Miller came to us with four hydraulic system concepts based on different pumps and circuit arrangements” said Scott Taylor, of Price Engineering, Hartland, Wis., an Eaton distributor “They asked us to evaluate each one and come back to them with a recommendation.”

The operational requirements for the hydraulic system were quite challenging. Service truck hydraulics typically power outriggers, winches, and cranes, plus various portable tools. Cranes and winches, in particular, had to provide precise control, regardless of the total load on the engine.

Depending on the manufacturer, these devices may use ei ther openor closed-center valving, both of which must be accommodated easily by the EnPak’s hydraulic system. Furthermore, because total engine output is limited to 27 hp — which mus t be divided among hydraulic pump, compressor, and generator to handle variable loads on each — an advanced flowcontrol capability had to be built into the hydraulic circuit.

“Our first response was designed around an Eaton Global gear pump with integral valving,” Taylor continued. “That looked like a cost-effective solution until we got more deeply involved in the dynamics of the application. It soon became apparent that the widely varying flow rates required to effectively manage engine output would need more compl excircuitry, which negated the savings typically realized when using a less sophisticated pump.

“Miller’s field research also determined that most of the hydraulicallypowered systems used on service trucks had capacities of either 10 or 20 gpm. That output range would be difficult to achieve with a fixed-displacement gear pump, and Miller did not want to build different EnPak power systems for high and low volume customers.

“It quickly become obvious that a variable-displacement piston pump was the best solution to Miller’s requirements,” Taylor said. “It could handle the dual displacement requirement with a simple mechanical displacement stop, and the infinitely variable displacement greatly simplified the flow metering required by the engine management system.”

Price Engineering recommended an Eaton 420 variable-displacement axial-piston pump with load sensing capability for the EnPak power system. The final system uses a Vickers EFV-1-12 series proportional throttle valve in the pump outlet to control flow based on input commands from the power management system. Miller’s testing showed that the Vickers valve had significantly lower hysteresis than other valves and provided consistent output under varying loads and pressures.

“We looked at various valves under heavy loads,” explained Miller’s design engineer Mark Peters. “The competitive valves weren’t pressure balanced like the Vickers valve and they would change flow rates under load, which impacted power management efficiency. End users need very smooth and precise hydraulic performance to operate the crane, and the stability of the Vickers valve accomplishes that.”

Dealing with cramped quarters “The very compact envelope of the Eaton 420 pump was another big plus. If you think about it, we were attempting to put what was traditionally a PTO, a hydraulic pump, a hydraulically driven air compressor, a stand-alone welder/generator, and a 400+ horsepower truck engine inside a box that measures 2 3 4 ft. That, obviously, placed a high premium on compactness of all the components.

“We had to make plumbing and clearance compromises to make a competitive pump we tested fit, but the 420 is compact enough to go completely inside without compromising anything. That’s really important in terms of packaging, and it helps us keep the EnPak very quiet as well as efficient. The end result to the user is a product that takes up less room on the truck and reduces overall weight.” “The 420 also has an extremely good input bearing design that can accommodate relatively high side loading. That let us power the pump with a simple belt and pulley system without an overhung load adapter. That saves space without sacrificing any reliability,” Peters added.

In addition to supplying Eaton 420 pumps and Vickers valves, Price also provides packaged hose assemblies for the Miller EnPak power system. In keeping with the premium performance criteria set for the EnPak power system, Price Engineering specified Aeroquip hoses that are cleaned internally after end fittings are installed to drastically reduce potential contamination.

“The hoses aren’t commodity products,” Peters noted. “They are standard Aeroquip offerings, but each one was carefully specified for the role it plays in the application. We had a lot of Aeroquip products to choose from, and we picked the best one for each use.”

Price Engineering fabricates the hose kits and keeps them in inventory for quick delivery to Miller Electric. The kits are customized for differing hose routing and connection options for open- or closed-center systems.

“Eaton’s distributor network was an important consideration to us,” said Miller product manager Chris Wierschke. “Obviously it means parts and service will be available locally for our customers, and that’s important.”